The totaln, offset, firstn, and nvalues parameters that are passed to the work function are useful for determining how much of the data has been processed and how much remains left to do. On the very first call to the work function firstn will be equal to offset + 1; the work function may need to perform various initialization tasks before starting to process the data. Similarly, firstn + nvalues - 1 will be equal to totaln on the last iteration, at which point the work function may need to perform some clean up operations before exiting for the last time. The work function can also force an early termination of the iterations by returning a status value = -1.
The narrays and iteratorCol.datatype arguments allow the work function to double check that the number of input arrays and their datatypes have the expected values. The iteratorCol.fptr and iteratorCol.colnum structure elements can be used if the work function needs to read or write the values of other keywords in the FITS file associated with the array. This should generally only be done during the initialization step or during the clean up step after the last set of data has been processed. Extra FITS file I/O during the main processing loop of the work function can seriously degrade the speed of the program.
One important feature of the iterator is that the first element in each array that is passed to the work function gives the value that is used to represent null or undefined values in the array. The real data then begins with the second element of the array (i.e., array, not array). If the first array element is equal to zero, then this indicates that all the array elements have defined values and there are no undefined values. If array is not equal to zero, then this indicates that some of the data values are undefined and this value (array) is used to represent them. In the case of output arrays (i.e., those arrays that will be written back to the FITS file by the iterator function after the work function exits) the work function must set the first array element to the desired null value if necessary, otherwise the first element should be set to zero to indicate that there are no null values in the output array. CFITSIO defines 2 values, FLOATNULLVALUE and DOUBLENULLVALUE, that can be used as default null values for float and double datatypes, respectively. In the case of character string datatypes, a null string is always used to represent undefined strings.
In some applications it may be necessary to recursively call the iterator function. An example of this is given by one of the example programs that is distributed with CFITSIO: it first calls a work function that writes out a 2D histogram image. That work function in turn calls another work function that reads the `X' and `Y' columns in a table to calculate the value of each 2D histogram image pixel. Graphically, the program structure can be described as:
driver --> iterator --> work1_fn --> iterator --> work2_fn
Finally, it should be noted that the table columns or image arrays that are passed to the work function do not all have to come from the same FITS file and instead may come from any combination of sources as long as they have the same length. The length of the first table column or image array is used by the iterator if they do not all have the same length.